I honestly thought the US already forced TikTok to store all data on a US company's servers.
It's the same reason why multiple data transfer frameworks between the EU and US have been ruled illegal (e.g. Privacy Shield).
US law does not care where the data of its companies is stored and allows for surveillance of non-US citizens. Since EU citizens have a right to privacy, no US company can legally handle EU-citizens data.
Obviously this is bad for trade and business, so there'll always be a new law made to temporarily allow for data transfers. Until it'll be ruled illegal again, and the cycle repeats.